The present invention relates to controlling the efficiency of combustion of fuel in a furnace and more particularly to the control of the efficiency of combustion of fuel in a furnace where the rate of flow of the fuel or the quality of the fuel may vary considerably over a period of time.
In general, a furnace has a fuel input, an air input and an exhaust output. The fuel and air, more specifically the oxygen in the air, are mixed and combusted within the furnace to liberate energy--mostly in the form of heat. The result of this combustion (chemical reaction) is energy and waste, for example carbon dioxide, and is removed through the exhaust output.
Fuel is typically hydrocarbons (chemicals composed of mostly carbon and hydrogen atoms). It has been long recognized from basic chemistry that for a given hydrocarbon a theoretical number of oxygen atoms is required for complete combustion of that hydrocarbon (e.g. a carbon atom requires two oxygen atoms to result in carbon dioxide). Since oxygen is a near constant proportion of air, the figure for the theoretical amount of oxygen can be transformed into a figure for the theoretical amount of air. Clearly, the furnace would not be operating efficiently if the amount of air into the furnace were below the theoretical amount. Fuel or combustibles, which can be translated into dollars and cents, would literally exit from the stack of the furnace. Moreover, this could create a very explosive condition, if the amount of combustibles were high.
On the other hand, it is not desirable to operate the furnace with an unlimited amount or excessive amount of air. Oxygen is only a small fraction (about 20%) of total air. Typically, air enters the furnace at ambient temperature of about 65.degree. F. At the exhaust output, the gaseous wastes, such as carbon dioxide, and the other gaseous components of air (mainly nitrogen) which do not enter into the combustion process, exit at an elevated temperature of about 350.degree. F. Thus, for every volume of air which is taken in at the air input, energy is wasted on about eighty percent of that volume of air in raising it to the elevated temperature at the exhaust output. It is known that for the most efficient operation of a furnace a limited amount of oxygen in excess of the theoretical amount of oxygen (or air) is required. Operation of the furnace above or below this excess amount of oxygen would cause the furnace to operate away from peak efficiency. However, the desired excess amount of oxygen for maximum efficient operation of the furnace varies as a function of the type and quality of fuel used. For example, natural gas may require only 2% excess oxygen for near peak efficient combustion while coal may require 8% excess oxygen.
After the combustion of fuel, the heat, which is liberated, is used for a variety of purposes, all of which can be generically termed as the load. A typical load is the use of heat to generate steam. Where the load is a constant, the amount of heat generated per unit time is also a constant. Consequently, the fuel flow rate is also a constant. Under such condition, the air flow rate can be adjusted, through trial and error, to obtain the most efficient operating point of the furnace for the particular fuel used.
In many industrial processes, however, the load is not a constant. Demand may vary by as much as 5% per minute in a typical paper processing plant. The variation in load would cause a variation in the heat produced per unit time. This can be accomplished by changing the fuel flow rate or by changing the type or quality of fuel used. In such environment, variations of such magnitude make the trial and error method totally useless.
Heretofore, one method of controlling the efficiency of combustion in a furnace is taught by U.S. Pat. No. 3,602,487 which uses an oxygen sensor at the stack (exhaust output) to detect the amount of oxygen leaving the stack. The amount of oxygen leaving the stack is excess oxygen, because the amount is more than that needed for complete combustion. The control of combustion based upon the detection of excess oxygen, however, would suffer the deficiencies as previously noted. Another method is taught by U.S. Pat. No. 3,723,047, which uses a combustible sensor to detect the combustibles level at the stack.